Method for manufacturing endless metal belt, endless metal belt, and belt-type continuously variable transmission

ABSTRACT

A method for manufacturing an endless metal belt used in a belt-type continuously variable transmission, wherein a stress-relief heat treatment is performed after the circumference of a ring body has been adjusted, and aging/nitridation is performed after the stress-relief heat treatment.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 14/418,104 filed Jan.29, 2015 (allowed), which is a national phase application based on thePCT International Patent Application No. PCT/JP2012/072142 filed on Aug.31, 2012, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a method for manufacturing an endlessmetal belt forming a power transmission belt to be used in a belt-typecontinuously variable transmission mounted in a vehicle, the endlessmetal belt, and the belt-type continuously variable transmission.

BACKGROUND ART

For instance, an endless metal belt to be used in a belt-typecontinuously variable transmission mounted in a vehicle and used totransmit drive power is composed of a plurality of endless metal rings(e.g., nine rings) stacked or laminated in close contact with eachother, the endless metal rings each having a circular-arc cross sectionand different circumferential lengths. In general, as a material of anendless metal ring, there is so far known maraging steel having superiorstrength characteristics. Further, an endless metal ring is made of asteel-strip annular member that has been subjected to rolling andthereafter to a solution treatment, a circumferential length adjusting(calibration), an aging-nitriding treatment in this order to enhancefatigue strength (for example, see Patent Document 1).

RELATED ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 2008-520437

SUMMARY OF INVENTION Problems to be Solved by the Invention

However, the circumferential length adjusting (calibration) is anexpanding work to slightly expand a ring body by looping the ring bodyover two rollers. After this work, the tensile residual stress and thecompressive residual stress are accumulated in the ring body. When thecircumferential length adjusting (calibration) is performed before theaging-nitriding treatment as in the technique of Patent Document 1,those residual stresses are not released in a later aging-nitridingtreatment and thus different residual stresses are distributed betweenthe outer peripheral side and the inner peripheral side of the ringbody.

FIG. 15 is a graph showing residual stress distribution in aconventional ring body. This graph shows measurement results of residualstress in an endless metal ring manufactured by undergoing the steps inthe conventional order by taking cross section of each ring body beforethe nitriding treatment and after the nitriding treatment by use of anX-ray stress measurement device to measure the residual stress from anouter peripheral surface to an inner peripheral surface. A horizontalaxis represents distance (μm) from the outer peripheral surface in thering body and a vertical axis represents residual stress (MPa). In thevertical axis, a minus side indicates compressive residual stress and aplus side indicates a tensile residual stress. From the above graph, itis found that before the nitriding treatment the compressive residualstress F0 is accumulated on the outer peripheral side of the ring bodyand a tensile residual stress G0 is accumulated on the inner peripheralside. The compressive residual stress F0 on the outer peripheral sidegradually decreases from the outer peripheral surface toward thevicinity of the center and becomes zero near the center and in turn thetensile residual stress G0 on the inner peripheral side graduallyincreases toward the inner peripheral surface.

The above graph shows that, after the nitriding treatment whereby anitrided layer has been formed with a predetermined depth (about 30 μm)from each of the outer peripheral surface and the inner peripheralsurface of the ring body, the compressive residual stresses E0 and D0are imparted on or close to the surfaces on the outer peripheral sideand the inner peripheral side. However, the temperature of the nitridingtreatment is on the order of 400 to 500° C. which is not so high as tomelt alloy constituents into solid solution. Thus, the compressiveresidual stress and the tensile residual stress caused by thecircumferential length adjusting (calibration) remain as-is left(corresponding to A0). Accordingly, as compared with a region (C0) wherethe nitrided layer on the outer peripheral side is absent, or ends, thetensile residual stress G0 previously accumulated by the circumferentiallength adjusting (calibration) on the inner peripheral side is much leftnear a region (B0) where the nitrided layer on the inner peripheral sideis absent, or ends. Thus, a portion (corresponding to B0) in which thetensile residual stress is left more than in other portions is likely tobecome a weakest portion apt to cause fatigue fracture when the ringbody is used in a continuously variable transmission.

To reduce the tensile residual stress mentioned above, there is known amethod of subjecting the surface of the endless metal ring toshot-peening work. This method further needs an additional stress-relieftreatment step, leading to an undesirable cost increase.

The present invention has been made to solve the above problems and hasa purpose to provide an endless metal belt manufacturing method, anendless metal belt, and a belt-type continuously variable transmission,capable of reducing tensile residual stress accumulated on an innerperipheral side by circumferential length adjusting to thereby enhancefatigue strength without adding a new stress-relief treatment step.

Means of Solving the Problems

(1) To achieve the above purpose, one aspect of the invention provides amethod for manufacturing an endless metal belt to be used in a belt-typecontinuously variable transmission, the method including: performing astress-relief heat treatment of a ring body after the ring body issubjected to a circumferential length adjusting work, and performing anaging-nitriding treatment of the ring body after the stress-relief heattreatment.

According to the above aspect, the ring body is subjected to thestress-relief heat treatment after the circumferential length adjustingwork, and then to the aging-nitriding treatment after the stress-reliefheat treatment. Accordingly, the ring body having therein littleresidual stress (the tensile residual stress and the compressiveresidual stress) generated due to the circumferential length adjustingwork can be subjected to the aging-nitriding treatment. Thus, thenitrided layer is formed with a predetermined depth on each of the outerperipheral side and the inner peripheral side of the ring body in whichresidual stress (the tensile residual stress and the compressiveresidual stress) is hardly left. The thus formed nitrided layers allowcompressive residual stress to be imparted nearly uniformly on the outerperipheral side and the inner peripheral side of the ring body. In otherwords, the magnitude of the compressive residual stress on the outerperipheral surface of the ring body and the magnitude of the compressiveresidual stress on the inner peripheral surface of the ring body becomealmost equal. Further, the magnitude of the tensile residual stress in aregion on the outer peripheral side of the ring body where the nitridedlayer is absent and the tensile residual stress in a region on the innerperipheral side of the ring body where the nitrided layer is absent alsobecome almost equal. Thus, in the region on the inner peripheral side ofthe ring body where the nitrided layer ends, there is formed no weakestportion having the tensile residual stress left more than in otherportions. Since the ring body in which the tensile residual stress andthe compressive residual stress are hardly left is subjected to theaging-nitriding treatment, the tensile residual stress accumulated bythe aging-nitriding treatment can be controlled to be nearly uniform andminimum in the wall thickness direction of the ring body.

(2) In the method for manufacturing an endless metal belt described in(1), preferably, the circumferential length adjusting work is performedafter the ring body is subjected to rolling work.

According to the above aspect, the ring body is subjected to rollingwork and then successively to the circumferential length adjusting work.Thus, the residual stress (the compressive residual stress and thetensile residual stress) by the rolling work and the residual stress(the compressive residual stress and the tensile residual stress) by thecircumferential length adjusting work can be simultaneously released byone stress-relief heat treatment. In other words, various residualstresses accumulated in the ring body until the end of thecircumferential length adjusting work can be simultaneously made almostzero in the single stress-relief heat treatment. Therefore, there is noneed to add another stress-relief heat treatment. The stress-relief heattreatment is different according to materials of the ring body. Forinstance, a material such as maraging steel or precipitation hardeningstainless steel will undergo a solution treatment, or solutionizing. Amaterial such as austenite stainless steel will undergo stress-reliefannealing. A material such as carbon steel (quench-hardened steel) willundergo a quenching treatment or both of a quenching treatment and atempering treatment.

(3) In the method for manufacturing an endless metal belt described in(1) or (2), preferably, the stress-relief heat treatment is performed ona plurality of ring bodies in a lamination state, each of the ringbodies having been subjected to the circumferential length adjustingwork.

According to the above aspect, the stress-relief heat treatment isperformed on a laminated body of the plurality of ring bodies eachhaving been subjected to the circumferential length adjusting. Theentire laminated ring bodies can provide enhanced rigidity, therebyenabling reducing the occurrence of undulation or deformation of an edgeface of each ring body during stress-relief heat treatment. Therefore,when used in the belt continuously variable transmission, the ringbodies can reduce the interference between an edge face of the endlessmetal belt in its width direction and neck portions of elements or theinterference between an edge face the endless metal belt in the widthdirection and wall surfaces of a V-shaped groove of a pulley. Further,since the ring bodies are laminated and then subjected to thestress-relief heat treatment, a heat treatment furnace can be greatlyreduced in size and thus reduce facility cost and energy cost.

(4) To achieve the above purpose, another aspect of the inventionprovides an endless metal belt manufactured by the method formanufacturing an endless metal belt as described in any one of (1) to(3).

According to the above aspect, it is possible to provide an endlessmetal belt with high fatigue strength at low cost by reducing tensileresidual stress on the inner peripheral side accumulated by thecircumferential length adjusting.

(5) In the endless metal belt described in (4), preferably, the ringbody has residual stress almost equally accumulated on an outerperipheral side and an inner peripheral side of the ring body.

According to the above aspect, the ring body or bodies have residualstress almost equally accumulated on the outer peripheral side and theinner peripheral side. Thus, stress load in a region where a nitridedlayer on the inner peripheral side corresponding to a conventionalweakest portion is absent can be reduced, leading to improved fatiguelife. To be concrete, it is more preferable that the magnitude of thecompressive residual stress on the outer peripheral surface of the ringbody and the magnitude of the compressive residual stress on the innerperipheral surface of the ring body become nearly equal, and, themagnitude of the tensile residual stress in a region on the outerperipheral side of the ring body where a nitrided layer is absent andthe magnitude of the tensile residual stress in the region on the outerperipheral side of the ring body where the nitrided layer is absentbecome nearly equal. This configuration can made the tensile residualstress accumulated in the ring body almost uniform and minimum in thewall thickness direction of the ring body and can accumulate thecompressive residual stress with good balance on the outer peripheralside and the inner peripheral side of the ring body to further improvethe fatigue life.

(6) To achieve the above purpose, still another aspect of the inventionprovides a belt-type continuously variable transmission including theendless metal belt as described in (4) or (5).

According to this aspect, it is possible to reduce the stress amplitudeon the endless metal belt during use and thus provide the continuouslyvariable transmission with long fatigue life at low cost.

Effects of the Invention

According to the invention, it is possible to provide a method formanufacturing an endless metal belt capable of reducing tensile residualstress on an inner peripheral side accumulated by circumferential lengthadjusting without adding another stress-relief treatment step to improvefatigue strength, the endless metal belt, and a belt-type continuouslyvariable transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical sectional view of a belt-typecontinuously variable transmission according to the invention;

FIG. 2 is a perspective view showing a part of an endless metal belt ina circumferential direction to be used in the belt-type continuouslyvariable transmission shown in FIG. 1;

FIG. 3 is a perspective view showing a part of an endless metal ringforming the endless metal belt shown in FIG. 2;

FIG. 4 is a diagram of a first manufacturing process for the endlessmetal belt according to the invention;

FIG. 5 is a diagram of a second manufacturing process for the endlessmetal belt according to the invention;

FIG. 6 is a process chart showing manufacturing processes for endlessmetal belts for comparison;

FIG. 7 is a perspective view of ring bodies subjected to a stress-reliefheat treatment in the first manufacturing process;

FIG. 8 is a perspective view of ring bodies subjected to a stress-reliefheat treatment in the second manufacturing process;

FIG. 9 is a perspective view of a carrying jig for conveying laminatedring bodies during an aging-nitriding treatment in the secondmanufacturing process;

FIG. 10 is a top view of the laminated ring bodies shown in FIG. 9;

FIG. 11 is a graph showing residual stress distribution in a ring bodyaccording to the invention;

FIG. 12 is a graph showing differences in residual stress distributionin ring bodies between a conventional art and the present invention;

FIG. 13 is a graph showing a warped amount of an edge face of the ringbody after the stress-relief heat treatment;

FIG. 14 is a graph showing fatigue life of an endless metal belt; and

FIG. 15 is a graph showing residual stress distribution in aconventional ring body.

MODE FOR CARRYING OUT THE INVENTION

A detailed description of a preferred embodiment of a method formanufacturing an endless metal belt, the endless metal belt, and abelt-type continuously variable transmission embodying the presentinvention will now be given referring to the accompanying drawings.

<Structure of a Belt-Type Continuously Variable Transmission and anEndless Metal Belt>

The structure of the belt-type continuously variable transmission andthe endless metal belt to be used therein will be first explained. FIG.1 is a schematic vertical sectional view of the belt-type continuouslyvariable transmission according to the invention. FIG. 2 is aperspective view showing a part of the endless metal belt in acircumferential direction to be used in the belt-type continuouslyvariable transmission shown in FIG. 1. FIG. 3 is a perspective viewshowing a part of an endless metal ring forming the endless metal beltshown in FIG. 2.

As shown in FIG. 1, a belt-type continuously variable transmission 100includes an input shaft J1 coupled to a drive source, an output shaft J2coupled to wheels, a drive pulley P1 and a driven pulley P2 which areprovided integral with the corresponding input shaft J1 and output shaftJ2 and are each formed with a V-shaped groove changeable in width, and apower transmission belt 10 wound in the V-shaped grooves of the drivepulley P1 and the driven pulley P2 and configured to transmit drivepower. This continuously variable transmission 100 is configured tochange the width of each V-shaped groove of the drive pulley P1 and thedriven pulley P2, thereby changing the winding diameter of the powertransmission belt 10 to continuously change a relative rotation speedratio (V/W) of the input shaft J1 and the output shaft J2.

As shown in FIG. 2, the power transmission belt 10 includes two sets ofendless metal belts 13 each of which is composed of a plurality ofendless metal rings 12 having different circumferential lengths andbeing laminated in close contact in a wall thickness direction, and aplurality of metal elements 11 having plate-like bodies arrangedannularly in the circumferential direction of the endless metal belts13, 13. It is to be noted that FIG. 2 shows three endless metal rings 12in a lamination state for simplifying the figure. The number oflaminated endless metal rings 12 is however not limited thereto and maybe nine or twelve.

On both sides of each metal element 11 in the width direction, beltholding grooves 114 are formed nearly horizontally, leaving a neckportion at the center. A lower end of the belt holding grooves 114 formsa saddle portion 112 contacting with inner peripheral surfaces of theendless metal belts 13. Inclination surfaces of the saddle portions 112inclining inward and downward from the end faces of the saddle portions112 in the width direction are drive transmission portions 113 infriction contact with the wall surfaces of each V-shaped groove of thedrive pulley P1 and the driven pulley P2 to transmit drive power. Themetal elements 11 transmit the drive power in such a manner that eachfront-side metal element 11 in the rotation direction is pushed by eachrear-side metal element 11 in the rotation direction. A boss 115 isformed on a front surface above each of the neck portions. Each boss 115is engaged in a recess not shown formed in a back surface above eachneck portion to prevent mutual positional displacement of thecontinuously arranged metal elements 11. The metal elements 11 can bemade of a high abrasion-resistant steel material, for example, carbontool steels (SK material).

As shown in FIG. 3, an outer peripheral surface 121 and an innerperipheral surface 122 of each endless metal ring 12 are formed in acrowning shape that is curved slightly upward at a center in section.The radius of the crowing shape is for example on the order of 800 mm.This crowning shape can facilitate keeping the lamination state. Eachedge face 123 of the endless metal ring 12 in the width direction isformed in a smooth shape. The edge faces 123 in the width direction areformed by barrel polishing or the like when burrs or the like areremoved from a ring body cut apart from a cylindrical body. The wallthickness t of each endless metal ring 12 is substantially constant andfor example can be on the order of 180 to 190 μm. In the presentembodiment, the material of the endless metal rings 12 is maraging steelthat necessarily contains iron, nickel, and molybdenum and alsoappropriately contains, as needed, cobalt, titanium, aluminum, andothers. The content of nickel in the maraging steel is not limited to 18to 19 weight % and could be on the order of 20 to 25 weight %. Thecontent of molybdenum is preferably at least 3 weight % or more.

<Method for Manufacturing the Endless Metal Belt>

The method for manufacturing the endless metal belt will be explainedbelow. This endless metal belt manufacturing method in the presentembodiment includes a method defined by a first manufacturing processand a method defined by a second manufacturing process.

(First Manufacturing Process)

A diagram of the first manufacturing process will be first explained.FIG. 4 is the diagram of the first manufacturing process for the endlessmetal belt according to the invention. As shown in FIG. 4, the firstmanufacturing process for the endless metal belt includes (a) acylindrical body forming step, (b) a joining step, (c) a firstsolutionizing (annealing) step, (d) a ring cutting step, (e), a rollingstep, (f) a circumferential length adjusting step, (g) a secondsolutionizing step, (h) an aging-nitriding treatment step, and (i) alaminating step. These steps are performed in the order of (a) to (i).

(a) The cylindrical body forming step is a step of forming a cylindricalbody 1 having a predetermined length in an axial direction and beingopen at both ends in the axial direction. In this step, a strip-shapedmaraging steel sheet Z is wound off from a coil, cut into a sheet ZS ofa predetermined size, and then bent to bring opposite edges intoabutment with each other. This bending work may include a method using aroll or a die. The wall thickness of the cylindrical body 1 is about 0.4to 0.5 mm and the diameter of the same is about 100 to 200 mm.

(b) The joining step is a step of joining the opposite edges of thecylindrical body 1 brought in abutment. This joining method may includea welding method by melting the opposite edges by plasma welding, laserwelding, or the like, a diffusion joining method by removing oxidizedfilm of the edges. In the present embodiment, the plasma welding methodis adopted. A welding device 2 is placed opposite a butting portion 14of the cylindrical body 1, and either the cylindrical body 1 or a nozzleof the welding device 2 is moved in the axial direction (an arrow Fdirection) to perform butt welding. A weld portion 21 is formed topenetrate from the outer peripheral surface to the inner peripheralsurface of the cylindrical body 1. When shrinkage or sinkage occurs at aboundary between the weld portion 21 and a base material portion 22, itleads to strength deterioration. Thus, welding conditions (spotdiameter, nozzle direction, welding speed, etc.) that cause no shrinkageare selected.

(c) The first solutionizing (annealing) step is a step of homogenizingthe hardness of the cylindrical body 1 that was partially hardenedduring the welding process in order to perform nearly uniform rolling inthe rolling step to be performed later. In this solutionizing step, thewelded cylindrical body 1 is put in an axially upright posture on a meshbelt or the like and then conveyed into a heat treatment furnace andsubjected to the first solutionizing. The first solutionizing is carriedout by heating the cylindrical body 1 to a temperature equal to orhigher than a temperature at which alloy constituents are dissolved in asolid solution, and holding the cylindrical body 1 for a required timeand then cooling it. If the hardness of the weld portion is in ahardness range corresponding to such a degree as to enable nearlyuniform rolling in the subsequent rolling step, the present step can beskipped.

(d) The ring cutting step is a step of cutting the cylindrical body 1having homogenized hardness, in a direction perpendicular to the axialdirection into a plurality of annular members 3 each having apredetermined length in the axial direction. This cutting method mayinclude a mechanical cutting method by making a blade edge of a cuttersequentially dig into the cylindrical body 1 in a circumferentialdirection, a thermally cutting method by melt cutting by use of a laseror the like, etc. Since irregularity (protrusions and depressions) suchas burrs generated in a cut area is likely to cause stress concentrationthereon during use, barrel polishing or the like is carried out toremove such irregularity and smoothen the surface of each annular member3.

(e) The rolling step is a step of rolling each polished annular member 3to be extended to nearly a wall thickness usable as an endless metalring 12. The rolling method may include for example a roller rollingmethod in which an annular member 3 is wound over two opposed rollers,and a third roller is provided to press the annular member 3 against oneof the two rollers and move in the circumferential direction to extendthe circumferential length of the annular member 3. The circumferentiallength of a rolled ring body 4 is about 600 to 700 mm.

(f) The circumferential length adjusting step is a step of adjusting thecircumferential length of each rolled ring body 4 to a predeterminedcircumferential length determined according to the order of laminationprior to forming the endless metal belt 13 by laminating a plurality ofendless metal rings 12. The circumferential length adjusting method mayinclude for example a roller adjusting method in which a ring body 4 iswound over two opposed rollers and applied with tensile force whilemeasuring a distance of one of the roller from the other roller. Errorsafter the circumferential length adjusting are controlled to about dozenor so μm to allow the ring bodies 4 to be laminated in close contactrelation.

(g) The second solutionizing step is a step of recrystallizing metaltexture of each ring body having the adjusted circumferential length torestore crystal structure of the metal texture deformed by the rollingwork and the circumferential length adjusting work, thereby relieving orremoving processing strain and internal stress (including residualstress). In this step, a second solutionizing treatment is performed bylocking the ring body after the circumferential length adjusting in aposture vertically extending in an axial direction on a carrying jig andconveyed into a heat treatment furnace. The second solutionizing isperformed by heating the ring body to a temperature equal to or higherthan a temperature at which alloy constituents of the ring body aredissolved in a solid solution, and holding the ring body for a requiredtime, and then cooling it. For example, the heating temperature is about820° C. and the holding time is about 2 minutes.

(h) The aging-nitriding treatment step is a step of subjecting each ringbody having the adjusted circumferential length and having undergone thesecond solutionizing to the aging treatment to precipitate alloyelements to ensure predetermined hardness and also to the nitridingtreatment to form a nitrided layer with a predetermined depth on a frontsurface side of the ring body to impart compressive residual stresstherein. The aging-nitriding treatment is carried out in a continuousfurnace. In particular, to allow nitriding gas to uniformly diffuse overthe surface of each ring body, a plurality of the ring bodies are lockedon a special carrying jig capable of arranging the ring bodies atintervals in the axial direction and passed through the continuousfurnace. A heat treatment history of each ring body is managed to allowlater checking.

(i) The laminating step is a step of laminating selected ones of thering bodies (endless metal rings 12) having been subjected to theaging-nitriding treatment, the selected ones whose circumferentiallengths are larger from one on the inner peripheral side toward anotheron the outer peripheral side, thereby forming an endless metal belt 13.When this belt 13 is used as the power transmission belt 10 in thebelt-type continuously variable transmission 100, the endless metalrings 12 forming the endless metal belt 13 could not receive uniformlythe stress exerted thereon in case the endless metal rings 12 are not inclose contact with one another. Accordingly, the endless metal rings 12have to be laminated in close contact with one another almost uniformlyover the entire circumference.

(Second Manufacturing Process)

A diagram of the second manufacturing process will be explained below.FIG. 5 is the diagram of the second manufacturing process for theendless metal belt according to the invention. FIG. 6 is a process chartshowing the manufacturing processes for endless metal belts forcomparison. FIG. 7 is a perspective view of a ring body having beensubjected to the stress-relief heat treatment in the first manufacturingprocess. FIG. 8 is a perspective view of an endless metal belt havingbeen subjected to the stress-relief heat treatment in the secondmanufacturing process. FIG. 13 is a graph showing a warped amount of anedge face of each ring body after the stress-relief heat treatment.

As shown in FIG. 5, the second manufacturing process for the endlessmetal belt includes (a) a cylindrical body forming step, (b) a joiningstep, (c) a first solutionizing (annealing) step, (d) a ring cuttingstep, (e) a rolling step, (f) a circumferential length adjusting step,(g) a laminating step, (h) a second solutionizing step, and (i) anaging-nitriding treatment step. These steps are performed in the orderof (a) to (i). In the method for manufacturing the endless metal beltaccording to present invention, differences between the firstmanufacturing process and the second manufacturing process are only inthat the laminating step of the ring body is performed after theaging-nitriding treatment (the first manufacturing process) or thelaminating step of the ring body is performed after the circumferentiallength adjusting (the second manufacturing process) as shown in FIG. 6.The details in each step (a) to (i) are explained above in the firstmanufacturing process and thus skipped hereinafter.

As shown in FIG. 6(b), in the first manufacturing process, after rollingand circumferential length adjusting, each ring body is subjectedseparately to the stress-relief heat treatment. Thus, the ring body 5having flat edge faces after the circumferential length adjusting may betransformed to a ring body 6 whose edge faces are undulated or deformedduring stress-relief heat treatment. On the other hand, as shown in FIG.6(c), in the second manufacturing process, after the rolling and thecircumferential length adjusting, a plurality of the ring bodies arelaminated in close contact with one another, thereby enhancing therigidity of the entire ring bodies, so that the laminated ring bodiesundergo the stress-relief heat treatment. Accordingly, as shown in FIG.8, the laminated ring bodies 7 after the circumferential lengthadjusting could be produced as ring bodies 8 whose edge faces are lesslikely to be undulated or deformed during the stress-relief heattreatment. To be concrete, as shown in FIG. 13, when the ring bodies aretreated one by one (Process 1 of the present invention: the firstmanufacturing process), the edge-face warped amounts of the ring bodiessubjected to the stress-relief heat treatment were about 0.1 to 0.4 mm.In contrast, when the ring bodies are treated in lamination form(Process 2 of the present invention: the second manufacturing process),the edge-face warped amounts of the ring bodies subjected to thestress-relief heat treatment were about 0.05 to 0.13 mm, with greatreduction of undulation or deformation during the stress-relief heattreatment.

In the aging-nitriding treatment step (i) in the second manufacturingprocess shown in FIG. 5, the carrying jig is adapted to uniformlydiffuse nitriding gas over the laminated ring bodies 7, as explainedbelow. FIG. 9 is a perspective view of the carrying jig for conveyingthe laminated ring bodies during the aging-nitriding treatment in thesecond manufacturing process. FIG. 10 is a top view of the laminatedring bodies shown in FIG. 9.

As shown in FIG. 9, for the aging-nitriding treatment in the secondmanufacturing process, a carrying jig 80 for conveying the laminatedring bodies 7 in a locked state includes a first support roller 81, asecond support roller 82, and a third support roller 83, respective axesC1, C2, and C3 being upright in a vertical direction, a rectangularsupport table 84 pivotally supporting a lower end of each of the supportrollers, and a carrying rack 85 that is arranged adjacent to the supporttable 84 and extends in a carrying direction. The first support roller81 includes cylindrical surfaces 812 contacting with the outerperipheral surfaces of the laminated ring bodies 7 and flanges 811contacting with the edge faces of the laminated ring bodies 7. Theflanges 811 are arranged more than one and spaced at predeterminedintervals in the axial direction. The second support roller 82 isprovided vertically and opposed to the first support roller 81 to clampthe laminated ring bodies 7 from the inner peripheral side thereof. Thesecond support roller 82 has a cylindrical surface 822 contacting withthe inner peripheral surfaces of the laminated ring bodies 7. The thirdsupport roller 83 is provided vertically in a position symmetric to thesecond support roller 82 with respect to the axis of the laminated ringbodies 7 and has cylindrical surfaces 832 contacting with the innerperipheral surfaces of the laminated ring bodies 7. The third supportroller 83 includes flanges 831 as many as the flanges 811 of the firstsupport roller 81 and at the same level of the corresponding flanges811. The diameter of the cylindrical surfaces 832 of the third supportroller 83 is smaller than the diameter of the cylindrical surfaces 812of the first support roller 81 and the cylindrical surface 822 of thesecond support roller 82. The first support roller 81 is provided at itslower end with a gear 814 engaging with a straight gear 851 of thecarrying rack 85. The second support roller 82 is provided with at itslower end with a gear 824 engaging with the gear 814.

As shown in FIGS. 9 and 10, when the support table 84 is moved in thecarrying direction (an arrow K direction), the first support roller 81is rotated in an arrow g direction, while the second support roller 82is rotated in an arrow h direction. The laminated ring bodies 7 clampedbetween the first support roller 81 and the second support roller 82 arethus rotated in an arrow i direction in association with rotation of thefirst support roller 81 and the second support roller 82. The thirdsupport roller 83 is rotated in an arrow j direction in association withrotation of the laminated ring bodies 7 in the arrow i direction.

At that time, each set of the laminated ring bodies 7 is formed withgaps S between the ring bodies around one side (near the third supportroller 83) opposed to the other side clamped between the first supportroller 81 and the second support roller 82. The position of the gaps Smoves in a circumferential direction of the laminated ring bodies 7 asthe support table 84 is moved in the carrying direction (the arrow Kdirection). Accordingly, in association with movement of the carryingjig, nitriding gas can be uniformly diffused over each one of thelaminated ring bodies 7.

<Residual Stress Distribution in the Ring Body and Fatigue Life of theEndless Metal Belt>

Examination results of residual stress distribution in the ring bodiesmanufactured by the aforementioned manufacturing processes using themaraging steel of the following components will be explained and furtherthe mechanism of improving the fatigue life of the endless metal beltwill be explained. FIG. 11 is a graph showing residual stressdistribution in the ring bodies according to the present invention. FIG.12 is a graph showing differences in residual stress distribution inring bodies between a conventional art and the present invention. FIG.14 is a graph showing fatigue life of the endless metal belt.

The alloy composition ratio (weight %) of maraging steel is defined asbelow: nickel (Ni) is about 17 to 19%, cobalt (Co) is about 7 to 13%,molybdenum (Mo) is about 3.5 to 4.5%, titanium (Ti) is about 0.3 to1.0%, aluminum (Al) is about 0.05 to 0.15%, and carbon (C) is 0.03% orless.

(Examination Results of Residual Stress Distribution in the Ring Bodies)

The graph shown in FIG. 11 is a graph showing measurement results ofresidual stress in each of the endless metal rings manufactured by themanufacturing method of the present invention, by taking the crosssection of each ring body before and after the nitriding treatment byuse of an X-ray stress measuring device to measure the residual stressfrom the outer peripheral surface to the inner peripheral surface. Ahorizontal axis represents direction (μm) from the outer peripheralsurface in the ring body and a vertical axis represents residual stress(MPa). In the vertical axis, a minus side indicates compressive residualstress and a plus side indicates tensile residual stress. From the abovegraph, before the nitriding treatment, it appears that the compressiveresidual stress is slightly accumulated on the outer peripheral side ofthe ring body, while the tensile residual stress is slightly accumulatedon the inner peripheral side. However, respective values of slightcompressive residual stress and slight tensile residual stress areapparent value appearing due to limitations of the measuring device.Specifically, since the ring bodies are each formed in a crowning shapeprotruding on the outer peripheral side, when a sample is fixed in themeasuring device, the crowning is corrected to be flat. Accordingly, ameasurement result shows as if the compressive residual stress isslightly accumulated on the outer peripheral side and the tensileresidual stress is slightly accumulated on the inner peripheral side. Itis however actually considered that the residual stress is nearly zerofrom the outer peripheral side to the inner peripheral side. The reasonwhy the residual stress could be made zero from the outer peripheralside to the inner peripheral side of the ring body is that thestress-relief heat treatment (the second solutionizing) performed beforethe nitriding treatment could restore the crystal structure of metaltexture deformed by the rolling work and the circumferential lengthadjusting work and substantially perfectly remove the processing strainand the internal stress (including the residual stress).

From the graph shown in FIG. 11, after the nitriding treatment, it isfound that the nitrided layer is formed with a predetermined depth(about 30 to 40 μm) from each of the outer peripheral surface and theinner peripheral surface of the ring body in the nitriding treatment.This reveals that this formed nitrided layer imparts nearly the samecompressive residual stresses E and D on or close to the surfaces on theouter peripheral side and the inner peripheral side. Further, thetensile residual stress A near the center in the wall thickness lessvaries and is almost uniform in the wall thickness direction. As aresult, a value of the tensile residual stress in a region (C) where thenitrided layer on the outer peripheral side is absent, or ends, and avalue of the tensile residual stress in a region (B) where the nitridedlayer on the inner peripheral side is absent, or ends, are nearly equalto each other.

The graph shown in FIG. 12 is a graph showing measurement results ofresidual stress in each of the endless metal rings manufactured by themanufacturing method of the conventional art and the manufacturingmethod of the present invention by taking the cross section of each ringbody after the nitriding treatment by use of an X-ray stress measuringdevice to measure the residual stress from the outer peripheral surfaceto the inner peripheral surface. A horizontal axis represents direction(μm) from the outer peripheral surface in the ring body and a verticalaxis represents residual stress (MPa). In the vertical axis, a minusside indicates compressive residual stress and a plus side indicatestensile residual stress. In the above graph, by comparison of thecompressive residual stress on the outer peripheral side in the ringbody, that in the present invention is smaller by about 180 MPa inapproximately parallel in an arrow P direction than that in theconventional ring body. By comparing a difference in compressiveresidual stress between the outer peripheral side and the innerperipheral surface, the conventional ring body provides a largedifference in compressive residual stress between the outer peripheralsurface and the inner peripheral surface that a value on the innerperipheral surface is lower by about 300 MPa than a value on the outerperipheral surface, whereas the ring body of the present inventionprovides an almost zero difference in compressive residual stressbetween the outer peripheral surface and the inner peripheral surfacethat a value on the inner peripheral surface is smaller by about 15 MPathan a value on the outer peripheral surface. By comparing thecompressive residual stress in a region where the nitrided layer isabsent on the inner peripheral side, furthermore, the ring body of thepresent invention is smaller by about 150 MPa in parallel in an arrow Qdirection than the conventional ring body. Consequently, in the ringbody of the present invention, the compressive residual stress is almostequal between the outer peripheral side and the inner peripheral side,and the tensile residual stress in the region where the nitrided layeris absent on the inner peripheral side greatly decreases than in theconventional ring body.

(Mechanism of Improving Fatigue Life of the Endless Metal Belt)

According to the manufacturing method of the present invention, thestress-relief heat treatment (the second solutionizing) is conductedbefore the nitriding treatment. Therefore, at a stage prior toconducting the nitriding treatment, the crystal structure of metaltexture deformed by the rolling work and the circumferential lengthadjusting work can be restored, so that the processing strain and theinternal stress (including the residual stress) can be substantiallyperfectly removed. Thus, the residual stress in the ring body from theouter peripheral side to the inner peripheral side can be made almostzero.

After the residual stress in the ring body from the outer peripheralside to the inner peripheral side is made almost zero, the nitridingtreatment is performed. Thus, almost the same residual stress is appliedon or close to the surfaces on the outer peripheral side and the innerperipheral side. The tensile residual stress near the center in wallthickness less varies and is nearly uniform in the wall thicknessdirection. As a result, a value of the tensile residual stress in theregion where the nitrided layer on the inner peripheral side is absentand a value of the tensile residual stress in the region where thenitrided layer on the inner peripheral side is absent are almost equal.This greatly reduces a portion which has locally increased stressamplitude and will be a weakest portion apt to cause fatigue breakagewhen the endless metal belt is used in the belt-type continuouslyvariable transmission.

In the present invention, furthermore, the ring body is subjected to thestress-relief heat treatment (the second solutionizing) in which theresidual stress in the ring body from the outer peripheral side to theinner peripheral side is made almost zero and then subjected to thenitriding treatment. Accordingly, it is unnecessary to add the tensileresidual stress previously accumulated by the rolling work and thecircumferential length adjusting work and thus possible to achieveuniformization and minimization of tensile residual stress.

Since the nitrided layer is formed after the residual stress of the ringbody is made almost zero before the nitriding treatment, as explainedabove, almost equal compressive residual stress is imparted on or closeto the surfaces on the outer peripheral side and the inner peripheralside and the tensile residual stress in the region where the nitridedlayer is absent can be uniformized and minimized. Consequently, when theendless metal belt 13 is used in the belt-type continuously variabletransmission 100, the stress load is greatly smaller than theconventional ring body, thereby enabling large improvement of fatiguelife (see FIG. 14).

The embodiment explained above may be changed in other specific formswithout departing from the essential characteristics of the presentinvention. For instance, the present embodiment uses maraging steel asthe material of the ring body but is not limited thereto. For example,precipitation hardening stainless steel, austenite stainless steel, andcarbon steel (quenched steel) may be usable. In this case, in the secondsolutionizing step shown in FIG. 4 or 5, the solution treatment isperformed for the precipitation hardening stainless steel, thestress-relief annealing is conducted for austenite stainless steel, orthe quenching treatment or both of the quenching treatment and thetempering treatment is performed for the carbon steel (quench-hardenedsteel).

INDUSTRIAL APPLICABILITY

The present invention is available as the method for manufacturing anendless metal belt forming a power transmission belt to be used in abelt-type continuously variable transmission mounted in a vehicle, theendless metal belt, and the belt-type continuously variabletransmission.

REFERENCE SIGNS LIST

-   1: Cylindrical body-   2: Welding device-   3: Annular member-   4, 5, 6: Ring body-   7, 8: Laminated ring body-   10: Transmission belt-   11: Metal element-   12: Endless metal ring-   13: Endless metal belt-   14: Butting portion-   21: Weld portion-   100: Belt-type continuously variable transmission

1. An endless metal belt to be used in a continuously variabletransmission, the endless metal belt being manufactured by an endlessmetal belt manufacturing method including: performing a stress-reliefheat treatment of a ring body after the ring body is subjected to acircumferential length adjusting work, and then, without performinganother circumferential length adjusting work, performing anaging-nitriding treatment of the ring body after the stress-relief heattreatment.
 2. The endless metal belt according to claim 1, wherein thecircumferential length adjusting work is performed after the ring bodyis subjected to rolling work.
 3. The endless metal belt according toclaim 1, wherein the stress-relief heat treatment is performed on aplurality of ring bodies in a lamination state, each of the ring bodieshaving been subjected to the circumferential length adjusting work. 4.The endless metal belt according to claim 1, wherein the ring body hasresidual stress almost equally accumulated on an outer peripheral sideand an inner peripheral side of the ring body.